A method for the operation of an inverter having reactive power capability which comprises a polarity-reversing method is known from DE 10 2014 102 000 B3. The inverter having reactive power capability operated under application of the known polarity-reversing method comprises a bidirectional DC/DC converter, a link circuit and an unfolding bridge, wherein the two poles of the link circuit are connectable in alternation to two terminals of an AC output by means of the unfolding bridge in order to alternate the polarity of the AC output with respect to the link circuit. This alternation takes place at the zero crossing of an alternating voltage present at the AC output. In the presence of a phase difference between the alternating current and the alternating voltage at the AC output, the direction of a current flowing through the link circuit is also reversed when the polarity of the AC output with respect to the link circuit is reversed between the half-waves of the alternating voltage using the unfolding bridge. If the alternating current lags the alternating voltage at the AC output, the AC output is disconnected from the link circuit during the reversal of the direction of the current flowing through the link circuit, and a freewheeling path is made available between the two terminals of the AC output. For this purpose, the two terminals of the AC output are short-circuited, in particular through bridge switches of the unfolding bridge. In order to reverse the direction of the current flowing through the link circuit, the link circuit is charged by a current flowing through a storage choke device of the DC/DC converter, and is discharged in the opposite direction after this current has decayed. In the case of alternating current leading the alternating voltage at the AC output, these measures taken when changing the polarity of the AC output with respect to the link circuit are not necessary, since for this purpose a DC voltage present at the link circuit acts as the driving force.
While the method known from DE 10 2014 102 000 B3 does avoid the high distortion factor that is associated with other methods for the operation of an inverter with reactive power capability with a DC/DC converter, a link circuit and an unfolding bridge, it nevertheless requires a bidirectional DC/DC converter as well as a mutual accurate matching of the link circuit and the storage choke device of the DC/DC converter. It is thus also not possible for a plurality of DC/DC converters to be connected in parallel with the DC link circuit in order, for example, to feed the electrical power of a plurality of photovoltaic generators operated with the respective DC/DC converters at their optimized working point into an AC grid through a common unfolding bridge.
A method for the operation of an inverter having reactive power capability, which comprises a polarity-reversing method is known from CN 103208935 A. The inverter having reactive power capability operated under application of the known polarity-reversing method comprises a bidirectional DC/DC converter, a link circuit and an unfolding bridge, wherein the two poles of the link circuit are connectable in alternation to two terminals of an AC output by means of the unfolding bridge in order to alternate the polarity of the AC output with respect to the link circuit. This alternation takes place at the zero-crossing of alternating current flowing through the AC output. The DC/DC converter comprises a step-down converter connected to a DC voltage source and a step-up converter acting in the opposite direction to it. The step-down converter switch, the step-up converter diode, and a choke used by both converters are each distributed over two components, so that a symmetrical arrangement results. The step-up converter is in this case connected to the DC voltage source through the step-up converter diode, distributed over two diodes, with a polarity opposite to that of the step-down converter. The step-down converter of the DC/DC converter is active when the alternating current and the alternating voltage at the AC output have the same arithmetic sign. If, however, the alternating voltage has its zero crossing before the zero crossing of the alternating current, then the step-up converter of the DC/DC converter is activated instead of the step-down converter. The link circuit is then connected with the DC voltage source via the step-up converter, while the polarity with respect to the connection of the link circuit with the DC voltage source is changed over through the step-down converter.
An inverter having reactive power capability for feeding electrical power of a DC voltage source into an AC grid comprising an unfolding bridge with two parallel half-bridge circuits as inverter bridges is known from document DE 10 2014 101 571 A1, wherein the connection of the output terminals of the unfolding bridge with the input terminals of the unfolding bridge is changed over at the zero crossing of the grid voltage. During a feed of reactive power in time segments of a grid period of the AC grid during which the inverter draws power from the AC grid, a switch of the inverter bridge is operated in such a manner that the electrical power drawn from the AC grid is converted into heat, in that this switch is placed into a linear operating mode, or is clocked dissipatively. In one embodiment, one of the half-bridge circuits provided to connect the output terminals of the unfolding bridge with the input terminals of the unfolding bridge is used for this purpose; in a further embodiment, an additional switch is provided for this purpose, arranged parallel to the half-bridge circuits between the input terminals of the unfolding bridge.